1. Field of the Invention
The present invention relates to anticounterfeiting security devices and methods employing holograms and/or encoded patterns of dots.
2. Description of Prior Art
It is well known that when a regular pattern of dots or lines is viewed through a transparent replica of that pattern, there is an interference between the pattern and the replica which results in Moire fringes. These fringes have a size and configuration that depends upon the spacing between the pattern and its replica, and the configuration of the dots and their arrangement in the pattern.
Methods for controlling Moire fringes to form detailed images are known in the prior art. Broken line patterns arranged to yield Moire fringes in the form of continuous-tone pictures, for the purpose of making anticounterfeiting devices have been described. In this prior art technique, a broken pattern of lines is viewed through a simple grating screen. The distorted pattern is a distortion of the grating screen, wherein the lines in the pattern are offset by an amount corresponding to the density of the picture which is to be encoded in the pattern. The use of such Moire fringes as a security device has been suggested in the prior art, but that approach has the fatal flaw of being too simple to simulate. Any skilled technician with darkroom experience and access to the standard tools of a graphics technician can counterfeit such patterns and decode messages encoded that way.
Methods have been described in the prior art for making holographic anticounterfeiting devices in the form of isolated dot holograms separated by materials with different chemical and physical characteristics, so that when the holograms are laminated into a sandwich construction or are overcoated, it is extremely difficult to remove the lamination or overcoating without destroying the dot holograms or their arrangement. While this is a useful technique to prevent direct copying of a security device, it is still somewhat subject to counterfeiting because it is not very difficult to reduce a hologram to dots if there is no need to be concerned with the precise locations of the dots.
Methods have also been described in the prior art for recording full-color holograms in the form of an array of dot holograms overlain with dot color filters. While such holograms are fairly difficult to manufacture, they have not been proposed for use as security devices and they do not per se have any covert features particularly useful for security devices.
No prior art method has suggested an anticounterfeiting or security system employing pseudorandom dot screens and/or pseudorandomly arranged dot holograms to encode messages for security purposes. Similarly, no prior art method has suggested an anticounterfeiting or security system employing modulated pseudorandom dot patterns to encode images.